[Cytokine and inflammation: role of IL-5 and its receptor system in inflammation]

The inflammatory response is mediated by various cytokines. IL-5 is one of the inflammatory cytokines. The study of IL-5 has its origins in the search for one of the B-cell differentiation factors, named T-cell-replacing factor (TRF), that induces antigen-stimulated B cells to differentiate into antibody-forming cells. Eosinophil-differentiation factor, EDF, is a factor produced by thoracic duct lymphocytes of parasite-infected rats. cDNA cloning and mAb against IL-5 enable us to identify this molecule as a cytokine (TRF/EDF) that has pleiotropic activity on various target cells besides B cells and eosinophils. The pleiotropic activity of IL-5 is directly dependent on the initial binding to the IL-5 receptor (IL-5R) on the target cell surface. IL-5 transduces its signals through high affinity IL-5R which is constructed by two distinct polypeptides, alpha and beta. IL-5R alpha binds IL-5 with low affinity and associates with the beta chain which can convert low affinity IL-5R to high affinity IL-5R. IL-5R beta that does not bind IL-5 by itself is essential for the IL-5 signaling and is shared among IL-5R, IL-3R, and GM-CSFR. These results imply why IL-5, IL-3, and GM-CSF are eosinophylopoietin. The truncated IL-5R alpha and IL-5R beta complexes can not transduce IL-5 signals, although they bound IL-5 with high affinity, suggesting that IL-5-specific signaling may be transduced through IL-5R alpha.